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AN INTRODUCTION to the P-ADIC NUMBERS Contents 1. Introduction AN INTRODUCTION TO THE p-ADIC NUMBERS ALEXA POMERANTZ Abstract. This paper introduces the p-adic numbers with an emphasis on comparison to the real numbers. It is mostly self-contained, but some basic knowledge in number theory, analysis, topology, and geometry is assumed. We begin by defining the p-adic metric and using this metric to construct Qp and Zp. We then move on to sequences and series and writing p-adic expansions. Lastly, we discuss topology and geometry in Qp. Contents 1. Introduction 1 2. The p-adic Metric 2 3. Construction of Qp and Zp 5 4. Sequences and Series in Qp 6 5. p-adic Expansions 8 6. The Topology on Qp 10 7. Geometry in Qp 12 Acknowledgments 14 References 14 1. Introduction The p-adic numbers, where p is any prime number, come from an alternate way of defining the distance between two rational numbers. The standard distance function, the Euclidean absolute value, gives rise to the real numbers. While the real numbers are more natural to most of us, this paper aims to present the p-adic numbers on an equal footing. For example, both fields are complete metric spaces. Unlike the real numbers, the p-adic numbers are an ultrametric space, leading to a number of fascinating but often counterintuitive results. The p-adic numbers are useful because they provide another toolset for solving problems, one which is sometimes easier to work with than the real numbers. They have applications in number theory, analysis, algebra, and more. One example is Hensel's lemma for finding roots of a polynomial. Another is Mahler's Theorem, a p-adic analog of the Stone-Weierstrass Theorem. Yet another is Monsky's Theorem, a theorem about triangulating squares whose proof makes use of 2-adic numbers. The reader is encouraged to read more about these in their own time, but the rest of this paper will be focused on introducing p-adic numbers more broadly. We start off by introducing the p-adic metric and then employing it to construct the p-adic numbers using Cauchy sequences. We then move on to some unique results about sequences and series that do not appear in real analysis. This leads 1 2 ALEXA POMERANTZ into a discussion of p-adic expansions, which will help us think about p-adic numbers in a more concrete way. Next, we discuss the topology on Qp to get a sense for p-adic spaces. Finally, we look at how p-adic geometry differs from Euclidean geometry. 2. The p-adic Metric We begin by defining a general notion of absolute value for an arbitrary field K. Definition 2.1. An absolute value on K is a function |·|: K ! R≥0 that satisfies the following properties for all x; y 2 K: (i) jxj = 0 if and only if x = 0; (ii) jxyj = jxjjyj, and (iii) jx + yj ≤ jxj + jyj (Triangle Inequality). Moreover, an absolute value is non-Archimedean if it also satisfies the following property: (iv) jx + yj ≤ maxfjxj; jyjg (Strong Triangle Inequality). An absolute value that does not satisfy property (iv) is Archimedean. Remark 2.2. We note that any function satisfying Property (iv) necessarily satisfies Property (iii). Definition 2.3. A metric on K is defined by a distance function d: K × K ! R≥0. An absolute value induces a metric defined by d(x; y) = jx − yj for all x; y 2 K. A set on which a metric is defined is called a metric space. A set with a metric induced by a non-Archimedean absolute value is called an ultrametric space. Lemma 2.4. Let K be an ultrametric space, and let x; y 2 K. If jxj= 6 jyj, jx+yj = maxfjxj; jyjg. Proof. Without loss of generality, we assume that jxj > jyj. Since K is an ultra- metric space, jx + yj ≤ maxfjxj; jyjg = jxj (Definition 2.1 and Definition 2.3). Also, x = (x + y) + (−y), so jxj ≤ maxfjx + yj; jyjg. Since jxj > jyj, jx + yj > jyj. Thus, we have jx + yj ≤ jxj ≤ jx + yj. It follows that jx + yj = jxj = maxfjxj; jyjg. Exercise 2.5. Check that in a field with a metric induced by an absolute value, the following properties hold for all x; y; z 2 K: • d(x; y) > 0 () x 6= y, • d(x; y) = d(y; x), and • d(x; z) ≤ d(x; y) + d(y; z). We now begin to define the p-adic metric specifically. For the rest of the paper, let p be a fixed prime number. Definition 2.6. The p-adic valuation on Q is defined by a function vp : Q ! Z [ f1g. Let x 2 Q where x 6= 0. If x 2 Z, let vp(x) be the unique positive integer satisfying v (x) 0 0 x = p p x , where p - x : a For all nonzero x 2 , we may write x = where a; b 2 . Then we define Q b Z vp(x) = vp(a) − vp(b): Lastly, we define vp(0) = +1. AN INTRODUCTION TO THE p-ADIC NUMBERS 3 Remark 2.7. We can think of the p-adic valuation as describing how divisible by p a number is. In this way, it makes sense to define vp(0) = +1 since 0 can be divided by p infinitely many times, yielding an integer (namely 0) each time. Exercise 2.8. Show that the p-adic valuation does not depend on the representa- a a0 0 0 tion of a rational number (if b = b0 , then vp(a) − vp(b) = vp(a ) − vp(b )). a0 Exercise 2.9. Prove that if x 2 , x = pvp(x) , where p a0b0. Q b0 - Lemma 2.10. The following properties hold for all x; y 2 Q: (i) vp(xy) = vp(x) + vp(y) and (ii) vp(x + y) ≥ minfvp(x); vp(y)g. a c Proof. Let x; y 2 . Then x = and y = , where a; b; c; d 2 . Q b d Z First, if x = 0 or y = 0, then vp(xy) = vp(x) + vp(y) = +1. We therefore consider the case that both x and y are nonzero. Then vp(x) = vp(a) − vp(b); and vp(y) = vp(c) − vp(d) (Definition 2.6): ac Moreover, since xy = , bd vp(xy) = vp(ac) − vp(bd): Since a; c 2 Z, v (a) 0 v (c) 0 0 0 a = p p a and c = p p c ; where p - a and p - c (Definition 2.6): Then ac = pvp(a)a0 · pvp(c)c0 = pvp(a)+vp(c)a0c0: 0 0 0 0 Since p - a and p - c , p - a c . Thus, by Definition 2.6, vp(ac) = vp(a) + vp(c). By a similar argument, vp(bd) = vp(b) + vp(d). It follows that vp(xy) = vp(ac) − vp(bd) = vp(a) + vp(c) − vp(b) − vp(d) = vp(x) + vp(y): Hence, the p-adic valuation satisfies the first property. Next, if x + y = 0, then vp(x + y) = +1 ≥ minfvp(x); vp(y)g trivially. We therefore consider the case that x + y 6= 0. Without loss of generality, we assume that vp(x) ≤ vp(y). Then vp(x) is finite (if this were not the case, then x + y = 0 since x = y = 0). By Definition 2.6, vp(x) = vp(a) − vp(b): Also, if y = 0, vp(x + y) = vp(x) = minfvp(x); vp(y)g: Thus, we assume that y 6= 0, so vp(y) = vp(c) − vp(d): Since vp(x) ≤ vp(y), vp(a) − vp(b) ≤ vp(c) − vp(d): It follows that vp(a) + vp(d) ≤ vp(b) + vp(c): Equivalently, vp(ad) ≤ vp(bc) (by part (i)). Since ad 2 Z and bc 2 Z, v (ad) v (bc) ad = p p m and bc = p p n; where p - m and p - n (Definition 2.6): 4 ALEXA POMERANTZ Then ad + bc = pvp(ad)m + pvp(bc)n: vp(ad) vp(bc) vp(ad) Since vp(ad) ≤ vp(bc), p j p . It follows that p j ad + bc. Thus, vp(ad + bc) ≥ vp(ad). We now manipulate this inequality as follows, applying the result from part (i) as necessary: vp(ad + bc) ≥ vp(ad) =) vp(ad + bc) ≥ vp(a) + vp(d) =) vp(ad + bc) − vp(d) ≥ vp(a) =) vp(ad + bc) − vp(b) − vp(d) ≥ vp(a) − vp(b) =) vp(ad + bc) − vp(bd) ≥ vp(a) − vp(b) ad + bc We recall that x + y = , so bd vp(x + y) = vp(ad + bc) − vp(bd): Thus, we have vp(x + y) ≥ vp(x) = minfvp(x); vp(y)g. In other words, the p-adic valuation satisfies the second property. Definition 2.11. Let the p-adic absolute value function |·|p : Q ! R≥0 be defined by ( p−vp(x) if x 6= 0, jxjp = 0 if x = 0. The p-adic absolute value induces the p-adic metric, denoted by dp. Remark 2.12. Since the p-adic valuation for nonzero rational numbers is always an integer, the p-adic absolute value takes a discrete set of values. Proposition 2.13. The p-adic absolute value is a non-Archimedean absolute value on Q. Proof. Let x; y 2 Q. First, we suppose that x = 0. Then jxjp = 0 by Definition −vp(x) 2.11. Now, we suppose that x 6= 0. Then jxjp = p by Definition 2.11. Since p 6= 0, jxjp 6= 0. Thus, the p-adic absolute value satisfies Definition 2.1(i). Next, we check the second property.
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